Aircraft structures are increasingly being constructed from a graphite epoxy composite material. It is often desirable to test samples of the material from each lot of material produced or purchased. The sheets of graphite fiber from which the composite material is produced are made by third parties. When the purchaser of the material receives a shipment including several lots of graphite fiber sheets, the purchaser often conducts tests to determine if the purchased graphite fiber meets the required specifications.
One method of testing the graphite fiber is to produce test specimens using specifications similar to the specifications that will be used in producing the material for use on the aircraft structure. After the specimens are prepared, they are subjected to various tests, including destructive tests. These tests include tensile tests in which the specimen is placed in tension and the load increased until the member fails. A compression test is also frequently performed in which the member is placed in compression and the load increased until the member fails. The amount of load held by the member until failure is an indication of the strength of the material. If the specimen fails under a small load this indicates that the sheets of material as purchased are defective, even though the proper steps in producing the final product were followed.
It is important that the testing of the specimens be accurate in all respects to ensure that the material is the safe for use on an aircraft. A problem in the prior art is difficulty of producing specimens that are uniform each time. The specimens are prepared from a larger sheet taken from the lot. This usually requires that the sheet be cut from the lot and other preparations performed on the sheet. It is possible to damage a specimen when cutting it from a larger sheet such that the load carried by the specimen is not indicative of the load that can be carried by that lot of material if properly prepared. Other specimens prepared from the same lot may fail under very different load conditions, thus indicating that procedures followed in preparing the specimens are important.